Poly(arylene sulfide) (PAS) resins are thermoplastic polymeric materials with good thermal stability, unusual insolubility, resistance to chemical environments, and inherent flame resistance. PAS resins additionally have good electrical insulative properties which make them ideal for electrical and electronic applications. Their excellent resistance to chemical degradation makes them ideal for use in chemical environments which involve organic solvents and strong mineral acids, such as coatings for pipes, tanks, pumps, and other equipment.
Poly(phenylene sulfide) (PPS) is a commercial product which is generally produced by reacting p-dichlorobenzene with sodium sulfide in a polar organic solvent to produce PPS and the by-product sodium chloride. This process is known as the Macallum polymerization procedure, and the basic process is disclosed in U.S. Pat. No. 2,513,188 and U.S. Pat. No. 2,583,941. An improvement on the Macallum polymerization procedure involves adding N-haloamides as catalysts in the procedure (U.S. Pat. No. 3,285,882). The Macallum polymerization utilizes only chloroaromatic compounds.
The PPS which is formed in the Macallum process has only a modest molecular weight on the order of 10,000-40,000 and has relatively low melt viscosity. Higher molecular weights can be obtained by heating the PPS in the presence of oxygen. During heating, the molecular weight of the PPS increases due to a variety of chemical reactions including oxidation, cross-linking, and chain extension. These curing reactions result in polymers which have inherent brittleness and reduced drawing capability while only achieving modest increases in molecular weight. Additionally, PPS which is produced by polymerization In the presence of sulfide and/or hydrosulfide salts, such as sodium sulfide and sodium hydrosulfide, has a residual content of inorganic salt present in the polymer. These residual salts are, for example, sodium chloride and sodium sulfide resulting from the combination of the sodium cation with chlorine or sulfide from the starting materials. The presence of these residual salts in the polymer increases the corrosive nature of the polymer and can cause a deterioration in the drawing or spinning characteristics of the polymer. Residual salts can also result in breakages in the spun fibers and additionally contribute to plugging and clogging of the spinnert holes.
An additional problem with poly(arylene sulfide) produced by the Macallum process is the effect of residual salts on the electrical properties. The presence of residual salts results in polymers with increased moisture adsorption and electrical activity, which are detrimental to applications requiring highly insulating characteristics. Although extensive extraction reduces the salt content of PPS produced by the Macallum process, complete removal of these salts is commercially infeasible.
An additional problem with PPS produced by the Macallum process is the high rate of crystallization of these polymers. Although some applications do require high rates of crystallization, many applications require much slower rates of crystallization. These polymers contain no substantial quantities of disulfide units.
Recently, poly(arylene sulfides) which do not contain substantial quantities of alkali metals have been discovered. These copolymers do not contain substantial quantities of alkali metals, simply because no alkali metal is used in the process to prepare them. They also have a variable rate of recrystallization, and contain small but substantial amounts of (--A--S--S--) units in the polymer chain.
The vast majority of units in these newly discovered polymers is (--A--S--) units, and the number of (--A--S--S--) or disulfide units is small compared to the number of (--A--S--) units. However, the number of (--A--S--S--) units in the copolymers is substantial, i.e. adequate for the instant invention.
Patents which relate to these newly discovered copolymers are U.S. Pat. Nos. 4,786,713; 4,792,600; 4,826,956; and 4,855,393, all assigned to Eastman Kodak Company. Copoly(arylene sulfide-disulfides) described in these patents are used as starting materials in the process of this invention.
In the process of this invention, advantage is taken of the disulfide linkages in the polymers described in the above-identified, recently issued patents. More specifically, such disulfide linkages are used as reactive sites to which chain transfer occurs, resulting in a new class of block copolymers.
Chain transfer mechanisms from growing polymer chains to disulfide linkages in dissimilar disulfide polymers have been described in the literature; Hallensleben, European Polymer Journal, Vol. 13, pp. 437-440 (1977), and Polymer Bulletin 1, 557-562 (1979). The prior art reaction has been conducted at low temperature (60.degree. C.) using polymers which are soluble and reactable at such temperatures. Such a process is not applicable to polyarylene sulfides made by the Macallum polymerization procedure, since those products do not have disulfide linkages. Applicant is the first to discover that radical chain transfer can take place using copoly(arylene sulfide disulfides) such as described in the recently issued patents cited above.